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    <title>ShellCheck: SC2099 – Use `$((..))` for arithmetics, e.g. `i=$((i + 2))`</title>
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    <h1>SC2099 – ShellCheck Wiki</h1>
    <a href="https://github.com/koalaman/shellcheck/wiki/SC2099">See this page on GitHub</a>
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    <h2 id="use--for-arithmetics-eg-ii--2">Use <code>$((..))</code> for
arithmetics, e.g. <code>i=$((i + 2))</code></h2>
<h3 id="problematic-code">Problematic code:</h3>
<div class="sourceCode" id="cb1"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb1-1"><a href="SC2099.html#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="va">i</span><span class="op">=</span>3</span>
<span id="cb1-2"><a href="SC2099.html#cb1-2" aria-hidden="true" tabindex="-1"></a><span class="va">i</span><span class="op">=</span>i <span class="ex">+</span> 2</span></code></pre></div>
<h3 id="correct-code">Correct code:</h3>
<div class="sourceCode" id="cb2"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb2-1"><a href="SC2099.html#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="va">i</span><span class="op">=</span>3</span>
<span id="cb2-2"><a href="SC2099.html#cb2-2" aria-hidden="true" tabindex="-1"></a><span class="va">i</span><span class="op">=</span><span class="va">$((i</span> <span class="op">+</span> <span class="dv">2</span><span class="va">))</span></span></code></pre></div>
<h3 id="rationale">Rationale:</h3>
<p>Unlike most languages, variable assignments in shell scripts are
space sensitive and (almost) always assign strings.</p>
<p>To evaluate a mathematical expressions, use <code>$((..))</code> as
in the correct code:</p>
<pre><code>i=$((i + 2))   # Spaces are fine inside $((...))</code></pre>
<p>In the problematic code, <code>i=i + 2</code> will give an error
<code>+: command not found</code> because the expression is interpreted
similar to something like <code>LC_ALL=C wc -c</code> instead of
numerical addition:</p>
<pre><code> Prefix assignment    Command  Argument
 LC_ALL=C             wc       -c
 i=i                  +        2</code></pre>
<h3 id="exceptions">Exceptions:</h3>
<p>If you wanted to assign a literal string, quote it:</p>
<pre><code>game_score=&quot;0 - 2&quot;</code></pre>
<h3 id="related-resources">Related resources:</h3>
<ul>
<li>Help by adding links to BashFAQ, StackOverflow, man pages, POSIX,
etc!</li>
</ul>
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